Document Id<Type&> and Id<Type&&> usage.

[BowenFu]

This is similar to ref (mut) patterns in rust.

[BowenFu]

Refer to https://github.com/BowenFu/matchit.cpp/blob/a6c34895db9e2c0b01a98c04664c637d5683b176/test/matchit/id.cpp for some samples.

Need to update REFERENCE.md and From-Rust-to-matchit.md.